The battery is your primary source of power to your computer. It is also where power is stored after being transferred via the charging cable so that it can be unplugged and used somewhere else for periods of time.
You may be reading this guide if you're having issues with the computer staying charged.
Slide the battery release all the way to the right.
Once the battery release is pulled all the way, the battery will lift itself slightly from the casing.
Remove the battery.